1. Field of the Invention
The present invention relates to a hybrid vehicle. More particularly, the present invention relates to a shift control system and a shift control method for a hybrid vehicle which provides an active kick-down shift according to operations of an engine in EV mode.
2. Description of Related Art
Because of demands for enhancement of fuel efficiency and reinforcement of exhaust gas regulations, eco-friendly vehicles have been researched. A hybrid vehicle is one type of such eco-friendly vehicles and attracts public attention.
The hybrid vehicle uses an engine and a motor as power sources, and enhances energy efficiency and reduces exhaust gas by selectively using the engine and the motor as the power sources. In order to minimize cost and torque loss, an engine clutch operated by fluid, instead of a torque converter, is mounted between the engine and the motor in the hybrid vehicle.
The engine clutch mounted in the hybrid vehicle connects or disconnects the engine and the motor according to a driving condition such that torque transmitted to the transmission is optimized.
Driving modes of the hybrid vehicle is divided into engine mode, driving by an engine, EV (Electric Vehicle) mode, driving by a motor, and HEV (Hybrid Electric Vehicle) mode, driving by an engine and a motor simultaneously.
Thus, how to harmonize power sources of the engine and the motor leads to enhancement of fuel efficiency.
In the conventional art, when a kick-down shift-request occurs for quick acceleration in the EV mode, a hybrid control unit (HCU) executes a kick-down shift during operation of a motor.
However, as shown in FIG. 4, since the more rotation speed of a motor is increased, the more torque is reduced, even if gear ratio is increase with a kick-down shift, sufficient acceleration may not be realized.
That is, motor rotation speed is increased with the kick-down shift but acceleration may be deteriorated and un-synchronized acceleration may happen.
In the conventional art, to solve the above problem, as shown in FIG. 5, when a kick-down shift from i shift to i-n shift is requested in the EV mode, a kick-down shift is prohibited and an engine is started using ISG (Idle Stop and Go).
After the engine start is completed, an engine clutch is engaged, and a kick-down shift of HEV mode is executed from the moment of power transmission of the engine.
However, above method also generates excessive acceleration dead zone time, which starts from the moment a kick-down shift is required to the moment the engine starts and the engine clutch is completely engaged, that is, real power transmission is too delayed.
Consequently, the conventional method may not improve acceleration response of a kick-down shift in EV mode of a hybrid vehicle.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.